1. Field of the Invention
The present invention relates to a process of forming metal images and, more particularly, to a process of forming substantially non-silver or low-silver content metal images at high sensitivity.
2. Description of the Prior Art
An electrophotographic process, a process wherein a photopolymer is used as the photosensitive material, and a process wherein a diazo compound is used as the photosensitive material have hitherto been employed as a process of forming images at comparatively high sensitivity or speed without using silver halide as the photosensitive material. However, these conventional non-silver type photographic processes are inferior in sensitivity to the silver halide photographic process and hence it is impossible to substitute the silver halide photographic process for these non-silver type photographic processes. In addition to these conventional photographic processes, an attempt toward increasing the sensitivity in photographic image forming process by physical development has also been made.
A physical development is a photographic development wherein an intensification or a growth of the development nucleus is achieved by treating the development nucleus with a developer containing a reducible metal compound and a reducing agent. In physical development, substantially all parts of the metal in the visible images formed are produced by the selective reduction of metal ions supplied from a reducible metal compound in a developer. Thus, it can be said that physical development be essentially the same as the so-called non-electrode plating.
A physical development wherein a silver compound is employed as the nucleus for the physical development is known and with regard to materials other than silver compounds usable as nuclei for physical development, noble metals such as gold, platinum, and palladium are known but materials other than these noble metals are not very well known. For example, a physical development wherein a noble metal is employed as the nuclei for physical development is described in the specification of U.S. Pat. No. 3,719,490, in which the physical development is carried out using a photosensitive palladium compound. The process disclosed in the above U.S. Patent is actually a useful image forming process but since the photosensitive component employed in the process is a photosensitive palladium compound, other photosensitive elements based on other photosensitive components than the photosensitive palladium compound cannot be used in the process and, also, the sensitivity of the photosensitive elements used in the process is generally low since the sensitivity thereof is determined by the sensitivity of the photosensitive palladium compound.
On the other hand, a photosensitive gold compound and a photosensitive platinum compound are also employed as the photosensitive component in physical development but since these noble metal compounds also have low sensitivity just as the photosensitive palladium compound, it is impossible to use these noble metal compounds as the nuclei for physical development with a high sensitivity as is the situation with silver halide. On the other hand, silver does not act as nuclei for physical development to materials other than silver.